In computers or data processing apparatuses, in order to realize a simple and intuitive operation, an input device that identifies a coordinate position that is pressed on a touch panel and inputs data has been used. A user presses a position corresponding to a button or an icon displayed on a screen so as to input operation information, and then enables to operate intuitively. Furthermore, a designer of the device is able to freely arrange buttons and icons on the screen.
In order to identify a pressed position on a touch panel, devices that detect the pressed position on the basis of a change in a resistance over a resistance film, a change in the electrostatic capacitance, and the like, have been used. However, in these methods, it is necessary to arrange a conductive material, such as a resistance film, on the front of a screen in order to detect the pressed position. For this reason, problems arise in that optical transmission is restricted, and the ease of viewing becomes poor.
On the other hand, there is a method in which a special material is not arranged on the screen, and a pressure detection sensor, such as a distortion gauge, is disposed so as to directly detect a pressure caused by pressing. Since a material that restricts the optical transmission is not arranged, there is a feature that ease of viewing is not decreased.
In such a method in which a pressure is detected to detect a pressed position, a detection value (reference output) output from a pressure detection sensor under a non-pressed state is stored as a reference level, and the actual pressure at the time of pressing is obtained on the basis of the difference between the reference level and the detection value at the time of pressing. Therefore, when the reference output is changed due to mechanical distortion, deterioration with age of a pressure sensor, and the like, there is a problem in that the pressed position cannot be correctly obtained. Accordingly, there is proposed, for example, “a touch position identification method for identifying a touch position of a touch on a touch screen, including a step of obtaining a plurality of reference levels for a force response touch signal; a step of selecting one or more reference levels from among the plurality of reference levels on the basis of the information obtained from the touch signal; and a step of identifying a touch position by using the selected one or more reference levels” (see, for example, Patent Literature 1). In this Patent Literature 1, one or more reference levels are used to compensate for a state that exerts an influence on a touch signal.